In recent years, the agricultural films produced from synthetic resins have been extensively used for green house cultivation or tunnel cultivation for field crops, etc. The agricultural films used for the above purposes are required on one hand to allow transmission of sunlight into the houses and tunnels with a high transmittance during day time, and on the other hand to absorb or reflect infrared rays emitted from plants and ground and thereby prevent the infrared rays from being released out of the houses and tunnels (i.e., have a good heat retaining property) during night time. It has been required that the heat retaining agent particles specifically used in the agricultural films are in the form of a compound comprising colorless ions having a refractive index in the vicinity of 1.45 to 1.55 as the refractive index of a resin in order to enhance transmission of sunlight through the agricultural films during day time, in the form of particles capable of exhibiting a high infrared absorption in the wavelength range of 5 to 30 μm which corresponds to an energy distribution of infrared radiation emitted from ground or plants, or in the form of fine particles having a good dispersibility so as not to cause deterioration in mechanical strength of the agricultural films when the particles are compounded therein.
To meet the above requirements of the heat retaining agents for agricultural films, there have been used hydrotalcite compounds. In order to allow the hydrotalcite compounds to exhibit an excellent transparency when added to resins, it is advantageous that the hydrotalcite compounds are in the form of particles having a small particle diameter. However, it may be difficult to uniformly disperse such particles having a small particle diameter in a master batch and a film, and there tends to arise such a problem that the particles have a poor dispersibility. In addition, as the particles become still finer, the bulk density thereof tends to be decreased, thereby causing problems such as poor handling property or poor compatibility with resins.
Conventionally, there has been proposed the method in which in order to improve a fluidity of hydrotalcite particles upon feeding the particles in a process for production of resin products, the bulk density of the hydrotalcite particles is well controlled owing to volume reduction of hydrotalcite by vacuum evacuation (Patent Document 1).
Also, there has been proposed the method in which hydrotalcite used as a heat retaining agent for agricultural films is coated with an oxysilicate compound or a phosphoric acid compound in order to impart still higher heat retaining property and transparency thereto (Patent Document 2).